The mixer or frequency changer stages of superheterodyne receivers change the frequency of an incoming Radio Frequency (RF) signal to a lower Intermediate Frequency (IF) signal. The consequent reduction of the frequency of operation of the stages subsequent to the mixer generally allows the superheterodyne receiver system to be superior to other receiver systems with respect to stability, gain and selectivity. The chief disadvantages of utilizing prior art mixer circuits for lowering the frequency of operation are the potential of increased internally generated noise, production of spurious responses and distortion.
More specifically, the mixers of superheterodyne receivers tend to create undesired signal outputs. This unique and unfortunate property is due to the ability of the mixer to generate sum and difference frequencies for all input signals that happen to be applied to the input terminal of the mixer. These input signals include the desired signals coming from mixer driving stages and sometimes undesired signals occurring in stages subsequent to the mixer which are fed back to the input terminal of the mixer. For instance, a problem sometimes created by prior art mixers in television receivers relates to "Channel 6 Beats." If the television receiver is tuned to Channel 6 the picture carrier and sound carrier occur at the mixer input terminal. The mixer adds the carrier component of the picture (83.25 MHz) to the sound carrier component (87.75 MHz) and subtracts this sum from the local oscillator frequency (129 MHz) to create a spurious signal having a frequency of about 42 MHz. The unwanted spurious 42 MHz signal then passes through the 42.17 MHz chroma processing channel and interfers with the quality of the color signal. Hence, the same second order nonlinearities of the mixer which enables it to translate the incoming signal to a desired lower intermediate frequency also enables prior art mixers to generate spurious signals which can fall within the desired pass band of the receiver. Furthermore, higher order mixer nonlinearities can provide other types of undesired mixer responses.
Prior art single device mixers also sometime suffer from other sorts of distortion problems. More specifically, cross modulation is another type of undesired or spurious mixer response which occurs because of the third order nonlinearities of the mixer device. The amount of undesired cross modulation is somewhat proportional to the local oscillator and RF signal power delivered to each active device of the mixer. Also, mixers employing only one device sometimes lack dynamic range. More specifically, single device mixers have a limited ability to accept local oscillator and RF input signals of high magnitudes without being driven into saturation or cutoff. Also, a further problem of prior art single device mixers is associated with the input impedance of the device changing with the local oscillator signal thereby causing driving source impedance distortion. In addition, some prior art unbalanced mixer circuits allows local oscillator and RF signals to be created at the output terminal thereof which pass into and cause distortion by undesired mixing action in subsequent stages.
Further problems associated with some prior art mixers relate to noise generation. Since mixing action normally occurs near the beginning of the signal stream of a receiver, any noise generated therein will be amplified by subsequent stages. Solid state mixing devices utilize P-N junctions which create noise. It is of course desirable to provide a mixer circuit which minimizes the effects of such noise.
Some prior art mixer circuits require balanced (double ended) drive. Such circuits generally are not suitable for use with present day television tuners and other driving circuits arranged to provide unbalanced (single ended) RF input signals. This is because such balanced drive mixer circuits require that more driving connections be switched when channels are changed than do the mixers with unbalanced input circuits. Accordingly, unbalanced mixer circuits used in television receivers, for instance, enable simplification of the present day tuner.
The current trend particularly in consumer electronics is to provide integrated circuits which include one or more stages hence introducing economy while at the same time providing excellent performance and improved reliability. For instance, it is desired to provide an integrated circuit that includes some combination of the RF amplifiers, oscillators, mixers and IF amplfiers for a television receiver. Some prior art mixer stages utilize field effect devices which are not suitable for being economically provided in bipolar integrated circuit form. Other prior art circuit configurations are too complex, and require too many external connections or components to be economically provided in integrated circuit form. Also, it is desirable in modern day television receivers to provide a Very High Frequency (VHF) mixer which can be easily changed to an IF amplifier for use at Ultra High Frequency (UHF) and which is manufacturable in integrated circuit form.